The immune system of mammals has evolved to protect the host against the growth and proliferation of potentially deleterious agents. These agents include infectious microorganisms such as bacteria, viruses, fungi, and parasites which exist in the environment and which, upon introduction to the body of the host, can induce varied pathological conditions. Other pathological conditions may derive from agents not acquired from the environment, but rather which arise spontaneously within the body of the host. The best examples are the numerous malignancies known to occur in mammals. Ideally, the presence of these deleterious agents in a host triggers the mobilization of the immune system to effect the destruction of the agent and, thus, restore the sanctity of the host environment.
The destruction of pathogenic agents by the immune system involves a variety of effector mechanisms which can be grouped generally into two categories: innate and specific immunity. The first line of defense is mediated by the mechanisms of innate immunity. Innate immunity does not discriminate among the myriad agents that might gain entry into the host's body. Rather, it responds in a generalized manner that employs the inflammatory response, phagocytes, and plasma-borne components such as complement and interferons. In contrast, specific immunity does discriminate among pathogenic agents. Specific immunity is mediated by B and T lymphocytes and it serves, in large, part, to amplify and focus the effector mechanisms of innate immunity.
The elaboration of an effective immune response requires contributions from both innate and specific immune mechanisms. The function of each of these arms of the immune system individually, as well as their interaction with each other, is carefully coordinated, both in a temporal/spatial manner and in terms of the particular cell types that participate. This coordination results from the actions of a number of soluble immunostimulatory mediators or “immune system stimulators” (reviewed in, Trinchieri et al., J. Cell Biochem. 53:301-308 (1993)). Certain of these immune system stimulators initiate and perpetuate the inflammatory response and the attendant systemic sequelae. Examples of these include, but are not limited to, the proinflammatory mediators tumor necrosis factors α and β, interleukin-1, interleukin-6, interleukin-8, interferon-γ, and the chemokines RANTES, macrophage inflammatory proteins 1-α and 1-β and macrophage chemotactic and activating factor. Other immune system stimulators facilitate interactions between B and T lymphocytes of specific immunity. Examples of these include, but are not limited to, interleukin-2, interleukin-4, interleukin-5, interleukin-6, and interferon-γ. Still other immune system stimulators mediate bidirectional communication between specific immunity and innate immunity. Examples of these include, but are not limited to, interferon-7, interleukin-1, tumor necrosis factors α and β, and interleukin-12. All of these immune system stimulators exert their effects by binding the specific receptors on the surface of host cells, resulting in the delivery of intercellular signals that alter the function of the target cell. Cooperatively, these mediators stimulate the activation and proliferation of immune cells, recruit them to particular anatomical sites, and permit their collaboration in the elimination of the offending agent. The immune response induced in any individual is determined by the particular complement of immune system stimulators produced, and by the relative abundance of each.
In contrast to the immune system stimulators described above, the immune system has evolved other soluble mediators that serve to inhibit immune responses (reviewed in Arend, Adv. Int. Med. 40:365-394 (1995)). These “immune system inhibitors” provide the immune system with the ability to dampen responses in order to prevent the establishment of a chronic inflammatory state with the potential to damage the host's tissues. Regulation of host immune function by immune system inhibitors is accomplished through a variety of mechanisms as described below.
First, certain immune system inhibitors bind directly to immune system stimulators and, thus, prevent them from binding to plasma membrane receptors on host cells. Examples of these types of immune system inhibitors include, but are not limited to, the soluble receptors for tumor necrosis factors α and β, interferon-γ, interleukin-1, interleukin-2, interleukin-4, interleukin-6, and interleukin-7.
Second, certain immune system inhibitors antagonize the binding of immune system stimulators to their receptors. By way of example, interleukin-1 receptor antagonist is known to bind to the interleukin-1 membrane receptor. It does not deliver activation signals to the target cell but, by virtue of occupying the interleukin-1 membrane receptor, blocks the effects of interleukin-1.
Third, particular immune system inhibitors exert their effects by binding to receptors on host cells and signaling a decrease in their production of immune system stimulators. Examples include, but are not limited to, interferon-β, which decreases the production of two key proinflammatory mediators, tumor necrosis factor α and interleukin-1 (Coclet-Ninin et al., Eur. Cytokine Network 8:345-349 (1997)), and interleukin-10, which suppresses the development of cell-mediated immune responses by inhibiting the production of the immune system stimulator, interleukin-12 (D'Andrea et al., J. Exp. Med. 178:1041-1048 (1993)). In addition to decreasing the production of immune system stimulators, certain immune system inhibitors also enhance the production of other immune system inhibitors. By way of example, interferon-α2b inhibits interleukin-1 and tumor necrosis factor α production and increases the production of the corresponding immune system inhibitors, interleukin-1 receptor antagonist and soluble receptors for tumor necrosis factors α and β (Dinarello, Sem. in Oncol. 24(3 Suppl. 9):81-93 (1997).
Fourth, certain immune system inhibitors act directly on immune cells, inhibiting their proliferation and function, thereby decreasing the vigor of the immune response. By way of example, transforming growth factor-β inhibits a variety of immune cells and significantly limits inflammation and cell-mediated immune responses (reviewed in Letterio and Roberts, Ann. Rev. Immunol. 16:137-161 (1998)). Collectively, these various immunosuppressive mechanisms are intended to regulate the immune response, both quantitatively and qualitatively, to minimize the potential for collateral damage to the host's own tissues.
In addition to the inhibitors produced by the host's immune system for self-regulation, other immune system inhibitors are produced by infectious microorganisms. For example, many viruses produce molecules which are viral homologues of host immune system inhibitors (reviewed in Spriggs, Ann. Rev. Immunol. 14:101-130 (1996)). These include homologues of host complement inhibitors, interleukin-10, and soluble receptors for interleukin-1, tumor necrosis factors α and β, and interferons α, β and γ. Similarly, helminthic parasites produce homologues of host immune system inhibitors (reviewed in Riffkin et al., Immunol. Cell Biol. 74:564-574 (1996)), and several bacterial genera are known to produce immunosuppressive products (reviewed in, Reimann et al., Scand. J. Immunol. 31:543-546 (1990)). All of these immune system inhibitors serve to suppress the immune response during the initial stages of infection, to provide advantage to the microbe, and to enhance the virulence and chronicity of the infection.
A role for host-derived immune system inhibitors in chronic disease also has been established. In the majority of cases, this reflects a polarized T cell response during the initial infection, wherein the production of immunosuppressive mediators (i.e., interleukin-4, interleukin-10, and/or transforming growth factor-β) dominates over the production of immunostimulatory mediators (i.e., interleukin-2, interferon-γ, and/or tumor necrosis factor β) (reviewed in Lucey et al., Clin. Micro. Rev. 9:532-562 (1996)). Overproduction of immunosuppressive mediators of this type has been shown to produce chronic, non-healing pathologies in a number of medically important diseases. These include, but are not limited to, diseases resulting from infection with: 1) the parasites, Plasmodium falciparum (Sarthou et al., Infect. Immun. 65:3271-3276 (1997)), Trypanosoma cruzi (reviewed in Laucella et al., Revista Argentina de Microbiolgia 28:99-109 (1996)), Leishmania major (reviewed in Etges and Muller, J. Mol. Med. 76:372-390 (1998)), and certain helminths (Riffkin et al., supra); 2) the intracellular bacteria, Mycobacterium tuberculosis (Baliko et al., FEMS Immunol. Med. Micro. 22:199-204 (1998)), Mycobacterium avium (Bermudez and Champsi, Infect. Immun. 61:3093-3097 (1993)), Mycobacterium leprae (Sieling et al., J. Immunol. 150:5501-5510 (1993)), Mycobacterium bovis (Kaufmann et al., Ciba Fdn. Symp. 195:123-132 (1995)), Brucella abortus (Fernandes and Baldwin, Infect. Immun. 63:1130-1133 (1995)), and Listeria monocytogenes (Blauer et al., J. Interferon Cytokine Res. 15:105-114 (1995)), and, 3) intracellular fungus, Candida albicans (reviewed in Romani et al., Immunol. Res. 14:148-162 (1995)). The inability to spontaneously resolve infection is influenced by other host-derived immune system inhibitors as well. By way of example, interleukin-1 receptor antagonist and the soluble receptors for tumor necrosis factors α and β are produced in response to interleukin-1 and tumor necrosis factor α and/or β production driven by the presence of numerous infectious agents. Examples include, but are not limited to, infections by Plasmodium falciparum (Jakobsen et al., Infect. Immun. 66:1654-1659 (1998); Sarthou et al., supra), Mycobacterium tuberculosis (Balcewicz-Sablinska et al., J. Immunol. 161:2636-2641 (1998)), and Mycobacterium avium (Eriks and Emerson, Infect. Immun. 65:2100-2106 (1997)). In cases where the production of any of the aforementioned immune system inhibitors, either individually or in combination, dampens or otherwise alters immune responsiveness before the elimination of the pathogenic agent, a chronic infection may result.
In addition to this role in infectious disease, host-derived immune system inhibitors contribute also to chronic malignant disease. Compelling evidence is provided by studies of soluble tumor necrosis factor receptor Type I (sTNFRI) in cancer patients. Nanomolar concentrations of sTNFRI are synthesized by a variety of activated immune cells in cancer patients and, in many cases, by the tumors themselves (Aderka et al., Cancer Res. 51:5602-5607 (1991); Adolf and Apfler, J. Immunol. Meth. 143:127-136 (1991)). In addition, circulating sTNFRI levels often are elevated significantly in cancer patients (Aderka et al., supra; Kalmanti et al., Int. J. Hematol. 57:147-152 (1993); Elsasser-Beile et al., Tumor Biol. 15:17-24 (1994); Gadducci et al., Anticancer Res. 16:3125-3128 (1996); Digel et al., J. Clin. Invest. 89:1690-1693 (1992)), decline during remission and increase during advanced stages of tumor development (Aderka et al., supra; Kalmanti et al., supra; Elsasser-Beile et al., supra; Gadducci et al., supra) and, when present at high levels, correlate with poorer treatment outcomes (Aderka et al., supra). These observations suggest that sTNFRI aids tumor survival by inhibiting anti-tumor immune mechanisms which employ tumor necrosis factors α and/or β (TNF), and they argue favorably for the clinical manipulation of sTNFRI levels as a therapeutic strategy for cancer.
Direct evidence that the removal of immune system inhibitors provides clinical benefit derives from the evaluation of Ultrapheresis, a promising experimental cancer therapy (Lentz, J. Biol. Response Modif. 8:511-527 (1989); Lentz, Ther. Apheresis 3:40-49 (1999); Lentz, Jpn. J. Apheresis 16:107-114 (1997)). Ultrapheresis involves extracorporeal fractionation of plasma components by ultrafiltration. Ultrapheresis selectively removes plasma components within a defined molecular size range, and it has been shown to provide significant clinical advantage to patients presenting with a variety of tumor types. Ultrapheresis induces pronounced inflammation at tumor sites, often in less than one hour post-initiation. This rapidity suggests a role for preformed chemical and/or cellular mediators in the elaboration of this inflammatory response, and it reflects the removal of naturally occurring plasma inhibitors of that response. Indeed, immune system inhibitors of TNF α and β, interleukin-1, and interleukin-6 are removed by Ultrapheresis (Lentz, Ther. Apheresis 3:40-49 (1999)). Notably, the removal of sTNFRI has been correlated with the observed clinical responses (Lentz, Ther. Apheresis 3:40-49 (1999); Lentz, Jpn. J. Apheresis 16:107-114 (1997)).
Ultrapheresis is in direct contrast to more traditional approaches which have endeavored to boost immunity through the addition of immune system stimulators. Preeminent among these has been the infusion of supraphysiological levels of TNF (Sidhu and Bollon, Pharmacol. Ther. 57:79-128 (1993)); and of interleukin-2 (Maas et al., Cancer Immunol. Immunother. 36:141-148 (1993)), which indirectly stimulates the production of TNF. These therapies have enjoyed limited success (Sidhu and Bollon, supra, Maas et al., supra) due to the fact: 1) that at the levels employed they proved extremely toxic; and 2) that each increases the plasma levels of the immune system inhibitor, sTNFRI (Lantz et al., Cytokine 2:402-406 (1990); Miles et al., Brit. J. Cancer 66:1195-1199 (1992)). Together, these observations support the utility of Ultrapheresis as a biotherapeutic approach to cancer—one which involves the removal of immune system inhibitors, rather than the addition of immune system stimulators.
Although Ultrapheresis provides advantages over traditional therapeutic approaches, there are certain drawback that limit its clinical usefulness. Not only are immune system inhibitors removed by Ultrapheresis, but other plasma components, including beneficial ones, are removed since the discrimination between removed and retained plasma components is based solely on molecular size. An additional drawback to Ultrapheresis is the significant loss of circulatory volume during treatment, which must be offset by the infusion of replacement fluid. The most effective replacement fluid is an ultrafiltrate produced, in an identical manner, from the plasma of non-tumor bearing donors. A typical treatment regimen (15 treatments, each with the removal of approximately 7 liters of ultrafiltrate) requires over 200 liters of donor plasma for the production of replacement fluid. The chronic shortage of donor plasma, combined with the risks of infection by human immunodeficiency virus, hepatitis A, B, and C or other etiologic agents, represents a severe impediment to the widespread implementation of Ultrapheresis.
Because of the beneficial effects associated with the removal of immune system inhibitors, there exists a need for methods which can be used to specifically deplete those inhibitors from circulation. Such methods ideally should be specific and not remove other circulatory components, and they should not result in any significant loss of circulatory volume. The present invention satisfies these needs and provides related advantages as well.